Which of the following is NOT a main influence on air density?

Air density is influenced significantly by a range of environmental factors, with temperature, pressure, and humidity being key components. Temperature affects air density because warmer air expands and becomes less dense, while cooler air contracts and is denser. Pressure also plays a crucial role—higher pressure compresses the air, increasing its density, while lower pressure allows air to expand and decreases its density. Humidity impacts air density as well; moist air is less dense than dry air because water vapor has a lower molecular weight compared to the nitrogen and oxygen it replaces.

While altitude does have a relationship with density, it is a consequence of the combined effects of temperature and pressure changes with increasing height in the atmosphere. As altitude increases, both pressure and temperature typically decrease, leading to lower air density. However, altitude itself is not a direct influence—it is the change in atmospheric conditions at different altitudes that affects density.

Understanding these interactions clarifies why temperature, pressure, and humidity are considered primary influences on air density, while altitude serves as a contextual factor derived from other influences.